While once thought to be cold blooded, after years of debate it is now accepted that the non-avian
dinosaurs were warmed blooded like modern birds. As birds are dinosaurs themselves, the closest
living relatives of the dinosaurs are the crocodilians, who are cold blooded like most other
reptiles. Therefore, it was previously assumed the dinosaurs’ warm-blooded metabolism evolved after
their last common ancestor with crocodilians. But while reasonable given the evidence available at
the time, additional evidence was since shown that assumption to be wrong. The first paper to provide
evidence for this, titled “Evidence for Endothermic Ancestors of Crocodiles at
the Stem of Archosaur Evolution'' was published in 2004. It hypothesized that,
like the dinosaurs, the ancestors of crocodilians were also warm blooded, only later reverting back
to a cold-blooded metabolism. Since then, more research has added to the original body
of evidence, confirming the hypothesis. This video will examine this evidence, from fossils
to evidence within living crocodilians. Part 2 will then explore how crocodilian thermoregulation
changed over time and why crocodilians reverted back to a lower, cold-blooded metabolism.
Before going any further, it is important to establish what the terms “warm blooded” and
“cold blooded” actually mean. “Cold blooded” is the popular term for an ectothermic
metabolism. The body temperature of ectothermic animals like crocodilians is derived from the
external environment. While this means their internal body temperature will vary wildly,
ectotherms can modify it through behavior, such as sitting in the shade to cool off or basking in the
sun to warm up. On the other end of the spectrum are the warm blooded, or endothermic species,
who generate their own body heat. Mammals and birds achieve this by burning vast amounts of
energy at the cellular level. This keeps their body temperature consistently at a narrow range,
which allows their internal biochemical reactions to always happen close to their optimal rate.
The result is that endothermic animals are able to be much more active than ectothermic ones.
It should be noted that this is an oversimplification. There are animals who have
an intermediate metabolism called mesothermy, with examples including monotremes, the naked mole-rat,
and the leatherback sea turtle. Some of these mesotherms modify their body temperatures in
ways other than burning energy. Additionally, some ectotherms have a higher metabolism than the norm,
with monitor lizards being one of the most extreme modern examples. Finally, there is no
fine line between ectothermy, mesothermy, and endothermy. To avoid overcomplicating things,
for the purpose of this video mesothermy will be considered a form of endothermy.
So, what traits are present in living crocodilians that suggest an endothermic ancestry?
While most reptiles have a three chambered heart, crocodilians instead possess a four
chambered heart, like birds and mammals. A four chambered heart allows for the separation of
oxygenated and deoxygenated blood, as well as systemic and pulmonary blood pressure.
This allows for a high systemic blood pressure while keeping the pulmonary blood pressure low.
A high systemic blood pressure is required for the high blood flow needed in endotherms,
though a high pulmonary blood pressure would be dangerous for both endotherms and ectotherms.
Therefore, without a four chambered heart, it would be impossible to safely maintain
a blood flow high enough to meet the high oxygen requirements of endothermic cells.
While clearly useful for endotherms, the advantages of a four chambered heart
for the cold-blooded crocodilians are less apparent. That said, crocodilian hearts do differ
from those of birds. Ectothermic reptiles can mix their oxygenated and deoxygenated blood to,
among other things, reduce oxygen consumption. Despite their four chambered hearts, crocodilians
can also mix their blood, but the site of this is not the heart like in other reptiles, but from a
complex set of outflow vessels. This suggests the modern crocodilian heart is the result of
them having to redevelop the ability to mix their blood after evolving their four chambered hearts.
Crocodilian limbs are also unusual. In general, ectotherms hold their legs in a sprawling posture
while endotherms have their limbs placed directly beneath their bodies. An upright posture allows
for the ability to both run and breathe at the same time, which is necessary for long periods
of activity. As for crocodilians, though they sometimes keep their legs in a sprawling position,
like other reptiles, they can also shift their legs into a semi- erect stance, called the “high
walk”, when they need to walk quickly. Some species of crocodiles take this even further,
being capable of galloping. While their semi-erect posture would still otherwise interfere with
breathing when running, their diaphragmaticus muscle allows for crocodilians to overcome this.
However, because of their low metabolism, crocodilians still lack the endurance to be
able to run like this for very long. Therefore, it is thought this behavior is an evolutionary
relic retained from more active ancestors. Additionally, the crocodilian respiratory system
exhibits unidirectional airflow like in birds, which is much more efficient than the respiratory
system of other reptiles and mammals. While not on par with that of their avian relations,
such a respiratory system is overbuilt for the needs of an ectotherm. Crocodilians in
particular hunt primarily by ambushing prey from the shore and, as previously stated,
are incapable of the long periods of activity than would select for such lungs. Therefore, like their
four chambered hearts and semi-erect limbs, their efficient respiratory system seems to have been
inherited from more active, endothermic ancestors. It is apparent that crocodilians are very atypical
for cold blooded reptiles. But does that prove that they were actually once endothermic?
Proving this required careful study of the fossils of their prehistoric ancestors.
Those reptiles with a more recent common ancestor with crocodilians than with dinosaurs are called
pseudosuchians. During the Triassic Period, which lasted from 250 million years ago to 200
million years ago, Pseudosuchia was very diverse. It included large carnivores,
armored herbivories, reptiles that converged upon a crocodilian-like body plan
independently of the actual crocodilians, and creatures that would have been best described
as dinosaur mimics if they hadn’t evolved before the dinosaurs in question. Within
Pseudosuchia is the clade Crocodylomorpha, which was the only branch of Pseudosuchia to survive
a mass extinction at the end of the Triassic period, and is the branch crocodilians belong to.
So, how do we know what the metabolism was of these long dead basal crocodylomorphs
and the other pseudosuchians? The easiest indicator is their limbs. Whereas crocodilians
can shift to a semi-erect posture, those of most other pseudosuchians, including many prehistoric
crocodylomorphs, were held directly beneath their bodies. This remained true in Crocodylomorpha
until the ancestors of the crocodilians transitioned to life in the water. This is exactly
what was predicted to be the case if crocodilians were the descendants of endotherms. However,
as will be elaborated on in Part 2, the limbs of some crocodylomorphs who were ectothermic,
if not to the same degree as most other reptiles, were also fully erect. Therefore,
though their erect limbs at the very least suggest endothermic ancestry, limb posture alone
is insufficient to determine a prehistoric animal’s basic metabolism. Perhaps the most
compelling evidence is the confirmation of prehistoric pseudosuchians doing something
ectotherms are incapable of. First, some pseudosuchians simply couldn’t survive as
ectotherms. Pushing blood vertically requires a high systemic blood pressure.
Which in turn requires a four chambered heart and a high cardiac output. Given the limits to how
active an ectothermic heart can be, this in turn limits the height of the highest point
of the body from their heart. While the bodies of today’s crocodilians conform to this limit,
some pseudosuchians from the Triassic period, such as Ornithosuchus and especially the sail-backed
Ctenosauriscus, exceeded it. Therefore, to actually pump blood throughout their bodies,
some of these Triassic pseudosuchians would have to have been somewhat endothermic. Of course,
it could be argued that these species evolved a high metabolism only after they split off from the
ancestors of the crocodilians. Fortunately, there is more evidence of pseudosuchian
thermoregulation. Fossilized bones can also show what a prehistoric animals growth rate was.
Endotherms are able to grow swiftly because their constantly high internal temperatures ensure the
chemical reactions required for growth happen as quickly as possible. Meanwhile, ectotherms,
even those in warm climates, take much longer to grow. Examination of the bones of pseudosuchians
to determine their growth rates found they were well above what ectotherms are capable of.
This included pseudosuchians very closely related to the crocodylomorphs, such as Postosuchus
and Batrachotomus. Indeed, Batrachotomus was able to reach its full 6-meter-long size in just
three years. For comparison, Nile crocodile males, who are smaller than Batrachotomus,
take at least twelve years to reach sexual maturity and even longer to reach their full size.
Bone growth rate can also be used to measure an animal’s resting metabolic rate. One study did
this by using the osteocyte shape, density, and area as well as the vascular density of the bone
as proxies. After examining the bones of various pseudosuchians, dinosaurs, and their distant
relations, it was found that a high metabolism was almost universal among Triassic pseudosuchians.
Overall, the evidence is conclusive that crocodilians are the descendants of endotherms
who later reverted to ectothermy. This evidence includes how their extinct ancestors and relatives
usually possessed a fully erect stance, an internal bone structure like those of endotherms,
growth rates impossible for ectotherms to achieve, indications of a high resting metabolic rate,
and in some cases bodies incapable of even being supported by an ectothermic metabolism.
Modern crocodilians also retain endothermic four chambered hearts modified to suit an ectothermic
metabolism, an overbuilt, a respiratory system with bird-like unidirectional airflow,
and a semi-erect stance. While the ancestors of most of today’s reptiles were no more
active than their modern descendants, the ancestors of the crocodilians were once
more like their famous dinosaurian cousins. However, this brings up more questions:
when did the ancestors of crocodilians revert to an ectothermic metabolism and why
did they do it? These two questions will be addressed in Part 2. I hope to see you then.